The present invention relates to apparatus for acquiring code synchronisation between a spreading code of a received spread spectrum radio signal and a local reference code. More especially the present invention relates to apparatus for acquiring code synchronisation between a spreading code of a received spread spectrum radio signal and a local reference code, wherein there are time and frequency offsets between the received spread spectrum radio signal and the local reference code.
Furthermore, the present invention relates to a method for acquiring code synchronisation between a received spread spectrum radio signal and a local reference code, and, more especially, a method of acquiring code synchronisation wherein there are time and frequency offsets between the received spread spectrum radio signal and the local reference code.
Spread spectrum communications systems are provided with means for spreading the spectrum of data bearing radio signals before transmission by transmitters operating within the system. The spectrum of the data bearing radio signal is spread by providing a means whereby the signal is arranged to modulate a spreading code. In the case of direct sequence spread spectrum systems, data carried by a radio signal is represented as a sequence of symbols which are arranged to modulate an n symbol spreading code wherein each symbol of the n symbol code is known as a chip. Therefore each data symbol of the direct sequence spread spectrum signal is represented as a modulated version of the n-chip code which has the effect of increasing the transmitted data rate and therefore the signal band width by a factor in dependence upon the value of n.
To detect and recover the data communicated by the data bearing radio signal a receiver within the spread spectrum system must be provided with a means for despreading the signal in accordance with the spreading code used by the transmitter. To this end the receiver is provided with a means for generating a local reference version of the n-chip spreading code and a means for correlating this n-chip spreading code with the received signal. However, in order for the receiver to correctly despread the radio signal the n-chip local reference code must be arranged to be synchronised with the corresponding n-chips of the spreading code of the received radio signal, such that n-chips of the locally generated reference code are temporally aligned and therefore correlated against the corresponding n-chips in the received signal.
Known systems for acquiring synchronisation of a spreading code operate to correlate n-chips of the local reference code with n samples of a received signal, and compare a result of the correlation with a predetermined threshold. The samples of the received signal are taken in accordance with a chip rate of the spreading code. Where the result of the correlation exceeds the predetermined threshold, code synchronisation is deemed to have been acquired. Where however the correlation result is below the predetermined threshold a shift of the received signal is introduced with respect to the local reference code such that correlation with the received signal takes place one chip later. The correlation is thereafter repeated for all n-possible shifts of the received signal with respect to the reference code until the correlation result exceeds the predetermined threshold at which point synchronisation is deemed to have been acquired.
A problem with known systems for acquiring code acquisition as hereinbefore described, is that timing uncertainty exists for a signal sample representative of a chip of the received signal since this signal sample may be representative of any temporal position within a chip period. The temporal position of a signal sample within a chip period will hereafter be referred to as the chip timing. Where the chip timing is offset from an optimum position by as much as a half of one chip, a resulting loss of energy in the correlation can result in a significant degradation in a probability of exceeding the predetermined threshold even where the n-chips of the local code is otherwise synchronised with the code sequence in the receiver.
Where a spread spectrum signal has travelled over a rapidly changing distance between a transmitter and a receiver of the spread spectrum system, frequency as well as timing uncertainty in a received signal may occur as a result of, for example, doppler frequency offsets. A receiver of a spread spectrum signal must therefore also resolve a frequency appertaining to a frequency offset uncertainty between a carrier frequency of the received signal and a local oscillator used to generate a signal for demodulating the received spread spectrum signal.
It is an object of the present invention to provide a receiver with improvements in acquiring code synchronisation.